1. Technical Field
This invention relates to the general field of compliant foil fluid film bearings and seals.
2. Background Art
Compliant foil fluid film radial bearings are currently being utilized in a variety of high speed rotor applications. These bearings are generally comprised of a bushing, a rotating element such as a rotor or shaft adapted to rotate within the bushing, non-rotating compliant fluid foil members mounted within the bushing and enclosing the rotating element, and non-rotating compliant spring foil members mounted within the bushing underneath the non-rotating compliant fluid foil members. The space between the rotating element and the bushing is filled with fluid (usually air) which envelops the foils. Conventionally, the compliant fluid foil elements are divided into a plurality of individual compliant foils to form a plurality of wedge shaped channels which converge in thickness in the direction of the rotation of the rotor.
The motion of the rotating element applies viscous drag forces to the fluid in the converging wedge channels. This results in increases in fluid pressure, especially near the trailing end of the wedge channels. If the rotating element moves toward the non-rotating element, the convergence angle of the wedge channel increases, causing the fluid pressure rise along the channel to increase. Conversely, if the rotating element moves away, the pressure rise along the wedge channel decreases. Thus, the fluid in the wedge channels exerts restoring forces on the rotating element that vary with and stabilize running clearances and prevent contact between the rotating and non-rotating elements of the bearing. Flexing and sliding of the foils against each other and against the bushing causes coulomb damping of any axial or overturning motion of the rotating element of the bearing.
Owing to preload spring forces or gravity forces, the rotating element of the bearing is typically in physical contact with the fluid foil members of the bearing at low rotational speeds. This physical contact results in bearing wear. It is only when the rotor speed is above what is termed the lift-off/touch-down speed that the fluid dynamic forces generated in the wedge channels assure a running gap between the rotating and non-rotating elements.
Compliant foil fluid film radial bearings typically rely on backing springs to preload the fluid foils against the relatively movable rotating element so as to control foil position/nesting and to establish foil dynamic stability. The bearing starting torque (which should ideally be zero) is directly proportional to these preload forces. These preload forces also significantly increase the rotor speed at which the hydrodynamic effects in the wedge channels are strong enough to lift the rotating element of the bearing out of physical contact with the non-rotating members of the bearing. These preload forces and the high lift-off/touch-down speeds result in significant bearing wear each time the rotor is started or stopped.
Conventional compliant foil fluid film radial bearings operate with extremely small running clearances and moderate, as opposed to low, drag and power consumption. The minimum clearances between the non-rotating fluid foil's converging channel ramp trailing ends and the rotating element are typically less than 100 micro-inches at operating conditions.
While most prior compliant foil fluid film radial bearings utilize a plurality of individual compliant foils and individual spring foils with a cylindrically bored bushing, there are instances where a single compliant foil and a single spring foil have been proposed with a cam shaped or lobed bushing. An example of this type of radial bearing can be found in U.S. Pat. No. 5,427,455 issued Jun. 27, 1995 to Robert W. Bosley, entitled “Compliant Foil Hydrodynamic Fluid Film Radial Bearing”, and assigned to the assignee of the present invention.
Another significant development in the design of compliant foil fluid film radial bearings is found in Weissert U.S. Pat. No. RE39,190 reissued Jul. 18, 2006. The Weissert patent discloses a design which utilizes inwardly projecting T-shaped retainers which allow the mounting of segmented compliant foils and foil undersprings in the internal bore of the bushing of the bearing.
It is also known that compliant foil fluid film radial bearings like those of the Bosley and Weissert patents inherently provide a significant sealing function to prevent the flow of fluids, typically gasses, in an axial direction parallel to the axis of the rotating shaft. Thus compliant foil fluid film radial bearings such as those of the Bosley and Weissert patents function both as bearings and seals. The various design parameters of the device can be modified to emphasize either its bearing function or its sealing function.
One example of a device specifically designed to function primarily as a compliant foil seal is shown for example in U.S. Pat. No. 6,505,837 issued Jan. 14, 2003 to Heshmat, entitled “Compliant Foil Seal”.
Compliant foil seals potentially are especially suitable for use in high speed high temperature turbomachinery such as gas turbine engines. Traditional seal technology typically used in gas turbine engines utilizes either brush seals which involve a plurality of closely packed bristles which engage the rotor outer surface, labyrinth seals which provide a plurality of grooves in the seal face which closely fits against the rotor, or plain seals which simply provide a close tolerance fit between a bore and a rotor.
In the field of brush seals and labyrinth seals it is also known to construct those seals in a segmented fashion wherein the seal can be disassembled into a plurality of arcuate segments which can then be assembled in place around the rotor of the turbomachinery. For example, segmented brush seals are shown in U.S. Pat. No. 5,110,033 issued May 5, 1992 to Noone et al., entitled “Segmented Brush Seal” and U.S. Pat. No. 6,913,265 issued Jul. 5, 2005 to Datta, entitled “Brush Seal Assembly, Method Of Manufacture And Use”. A segmented labyrinth seal is shown for example in U.S. Pat. No. 6,588,764 issued Jul. 8, 2003 to Fuller, entitled “Segmented Labyrinth Seal Assembly And Method”.
All of the patent references cited above are incorporated herein by reference.